1. Field of the Invention
The present invention generally relates to the field of fiber optic cables, in particular the present invention is directed to a method and apparatus for applying water barrier gels to optical fibers or fiber bundles at high speeds.
2. Discussion of Related Art
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with extremely low signal loss as compared to standard wire or cable networks. Optical fiber has found increasingly widespread application and currently constitutes the backbone of the worldwide telecommunication network. Because of this development, there has been a growing need for better quality optical fibers with a decrease in production time and costs, while ensuring adequate material strength for continued operation in increasingly harsh conditions. An important aspect for making better optical fibers is the reduction of structural faults or impurities in the protective coatings applied to the optical fiber during manufacture.
In general, optical fibers are manufactured from relatively large diameter glass preforms. Fiber optic preforms are generally made with three concentric glass layers. The inner layer, or core, is made of a very high quality, high purity SiO2 glass, which for example, may be about 5 mm in diameter. This high purity core is the portion of the optical fiber in which the optical data is transmitted. Concentrically positioned around the high purity core is a second layer of glass, or cladding, with a lower index of refraction then the inner core, and generally is less pure. The difference in refraction indices between the core and cladding allows the optical signals in the core to be continuously reflected back into the core as they travel along the fiber. The combination of the core and cladding layers is often referred to as the “primary preform.” The optical fiber is then formed by heating and softening a portion of the preform, and rapidly drawing the softened portion with specialized equipment. The length of the drawn optical fiber is typically several thousands of times the length of the primary preform. Optical fibers intended for manufacture of telecommunications cables are typically coated with one or more polymer layers. The polymers provide mechanical protection of the fiber surface, and are colored for identification purposes. The coated optical fibers, singly or in groups, are typically covered with one or more of a number of jackets that provide structural support and environmental protections. The aggregate of the optical fiber, jackets, and additional integrated mechanical supports, is typically referred to as an optical fiber cable.
Exposure to water or humid air causes chemical changes in the surface of the optical fiber, resulting in a degradation of its ability to carry information. The most common method used to prevent or mitigate this degradation, is to reduce or eliminate water contact on the fiber surface by substantially filling the protective housings with a water barrier compound such as a hydrophobic fluid. For a number of reasons, including cable behavior during installation and long-term stability of the cables during use, the hydrophobic fluid is typically a gel. Gels tend to flow when mechanically stressed, but tend to remain static when under a low mechanical load.
Known methods for applying gel to fibers include drawing the fibers through a reservoir filled with gel so that the fibers are coated. However, the use of such a method often results in an inconsistent coating on the fibers due to air entrapped air. Accordingly, gel applicators have been developed, such as the device disclosed in Griser et al. U.S. Pat. No. 5,395,557, which attempts to reduce air entrapment by using a reservoir filled with pressurized gel. This device includes a housing having a cavity through which a plurality of separated optical fibers are fed. Gel is provided to the cavity from a gel reservoir via a pump. The optical fibers are then drawn through the gel so that the fibers are coated with the gel. The gel is provided under pressure in an attempt to reduce air gaps that may form upon the fibers. However, this technique has numerous draw backs. For example, a relatively large driving pressure is placed upon the gel in the reservoir to reduce air entrainment. Rapid application of barrier gel with this method requires relatively long and narrow application regions to prevent uncontrolled ejection of fluid from application regions, due to the large pressures.
Consequently, an apparatus for applying gel to a plurality of optical fibers, which substantially overcomes the above-recited drawbacks is highly desirable and needed in the optical fiber industry.